Complexes of H1 histone and closed circular SV40 DNA were analyzed on sedimentation velocity gradients. At 0.1 M NaCl the binding of H1 to relaxed DNA is cooperative, yielding only rapidly sedimenting complexes containing an average of 40-45 molecules of H1 per molecule of relaxed DNA. Under the same conditions superhelical DNA forms two distinct complexes, a slowly sedimenting one containing 10-12 molecules of H1 per molecule of DNA and a rapidly-sedimenting, cooperatively formed complex containing between 45 and 80 molecules of H1 per molecule of DNA. Phosphorylated H1 histone forms similar complexes. Complexes of superhelical DNA with histones H3 and H4 were also studied, using DNA relaxing enzyme, hydrodynamic measurements, and electron microscopy to study the effect of H3 and H4 on DNA conformation. It was found that H3 and H4 alone compact duplex DNA and introduce torsional constraints equivalent to that obtained with the full complement of histones H3, H4, H2a, and H2b, in intact nucleosomes. BIBLIOGRAPHIC REFERENCES: Bina-Stein, M., Vogel, T., Singer, D.S., and Singer, M.F.: H5 histone and DNA-relaxing enzyme of Chicken Erythrocytes. J. Biol. Chem. 251: 7363-7366, 1976. Bina-Stein, M., and Singer, M.F.: The effect of H1 histone on the action of DNA-relaxing enzyme. Nucleic Acids Res. 4: 117-127, 1977.